There are conventional devices that carry out multi-step analytical procedures in an automated or semi-automated fashion. For example, microbiological analytical systems currently carry out automated antimicrobic susceptibility testing procedures using both photometric and fluorometric detection methods. The MicroScan Division of Baxter Healthcare Corporation sells a device of this type under the trade designation "Walk-Away." Armes et al. U.S. Pat. No. 4,676,951 and Hanaway U.S. Pat. Nos. 4,643,879 and 4,681,741 describe certain features the Walk-Away system.
Prior commercial embodiments of the Walk-Away system analyze trays carrying microbiologic specimens. The system includes an enclosed incubation chamber for the specimens. The system adds reagents to the specimens and analyzes them. All these activities take place within the incubation chamber.
Prior commercial embodiments of the Walk-Away system house the reagent fluids outside the incubation area. The system conveys these fluids under positive pressure through relatively long stretches of tubing leading from the reagent source to the incubation area. A mechanism within the incubation area manipulates the nozzle that dispenses the reagents.
Prior commercial embodiments require about 8 pounds per square inch of positive pressure to convey the reagents through the tubing into the dispensing site within the incubation area. Due to the length of the tubing and the amount of positive pressure that must be applied, the reagent fluids sometimes release entrapped air. The released air forms air pockets that block fluid flow in the tubing. This undesirable result is called "outgassing."